Pex for fun - from Microsoft Research

1,257,058 clicked 'Ask Pex!'

Does the Parameterized Unit Test pass for all input values? Click Ask Pex! to find out.

using System;
using Microsoft.Pex.Framework;
[PexClass]
public class TestClass {
  // Which values will trigger collisions in MyHashSet? Ask Pex to find out!
  [PexMethod] // this puzzle is a 'Parameterized Unit Test'
  public void TestAddContains(int x, int y) {
    var s = new MyHashSet(); 
    s.Add(x);
    s.Add(y);
    PexAssert.IsTrue(s.Contains(x));
    PexAssert.IsTrue(s.Contains(y));
  }
}
class MyHashSet {
  // a simple implementation of a hashtable, fixed in size, slightly buggy
  int[] buckets = new int[13];
  private int getInitialBucketIndex(int value) {
    return (value ^ 101010101) % buckets.Length;
  }
  public void Add(int value) {
    if (value==0) throw new ArgumentException("'0' not allowed");
    var index = getInitialBucketIndex(value);
    // this loop deals with collisions
    while (buckets[index] != value && buckets[index] != 0) {
      index = (index + 1) % buckets.Length;
      Console.WriteLine("[DEBUG INFO] collision in Add at index {0}", index);
    }
    buckets[index] = value;
  }
  public bool Contains(int value) {
    if (value == 0) throw new ArgumentException("'0' not allowed");
    var index = getInitialBucketIndex(value);
    while (buckets[index] != value && buckets[index] != 0)
      index = (index + 1) % buckets.Length;
    return buckets[index] == value;
  }
}